The problems and needs that form the basis of the present invention are primarily—but not exclusively—related to such turning tools that make use of round turning inserts. Such turning inserts have a basic shape that is determined by a rotationally symmetrical clearance surface around a central axis, which may be conical as well as cylindrical, and extends between, on one hand, a top side or chip surface, and on the other hand an underside. In the transition between the chip surface and the clearance surface, there is a cutting edge that is endless and circular. During operation, only a certain part of the cutting edge is in engagement with the workpiece, viz. the arc part that, depending on the diameter of the turning insert and the cutting depth in question, generates a cut surface in the workpiece. In other words, the arc length of the active edge part is small when the cutting depth is small, and greater when the cutting depth is greater.
The round turning inserts of the prior art had a simple, planar underside and were fixed in an insert seat having a planar bottom and a rear, round support surface against which a rear part of the clearance surface could be pressed. Recently, round turning inserts have been developed, which on the underside thereof have a coupling element in the form of a single, cross section-wise V-shaped engagement member (being the lower material portion in the turning insert), which engages an analogously cross section-wise V-shaped seating in the insert seat of the basic body of the tool. Although such turning inserts, in theory, give a more stable fixation than the simpler, older turning inserts, in practice, the same are associated with disadvantages. One such disadvantage is that the desired stability improvement is of a theoretical character. Hence, an exceptionally high accuracy is required for the two V-shapedly inclined male flanks of the turning insert, as well as the complementary female flanks in the insert seat, in order for the male flanks of the turning insert to be pressed in close surface contact against the female flanks. In this connection, it should be pointed out that the turning inserts and the basic body of the tool are manufactured on different sites and from different materials. While the turning inserts are manufactured from extremely hard, usually compression-molded and sintered materials, such as cemented carbide, ceramics, cement or the like, the basic body is manufactured from a softer material, usually steel. In the manufacture of, in particular, the turning inserts, form defects may arise, which usually are utmost small, for example of the order of a few thousandths of a millimeter, but which in practice means that the two male flanks of the turning insert do not get complete surface contact with the two female flanks of the insert seat. In other words, the turning insert will wobble or be over-determined in the insert seat.
Another disadvantage of the known turning inserts is that the same only can be indexed into two positions, viz. by being rotated 180° in such a way that two diametrically opposed edge portions having a limited arc length become active. Frequently, the turning inserts are used only for small cutting depths. This means that only two very short edge portions become worn, while great parts of the endless edge as to the rest remain unutilized. Thus, it may occur that the turning inserts are worn only along 90° of the circumference thereof, while 270° remain unutilized.
When the turning inserts are small (the diameter <15 mm) and the insert-carrying part of the basic body is comparatively weak, there is furthermore a certain risk that the wedge-shaped bottom part of the turning insert tends to crack or cleave the insert seat, because the cutting forces principally propagate axially through the turning insert.
In DE 3448086 C2, a tool intended for turning, more precisely internal turning, is disclosed, which is composed of a elongate, shaft-like basic body and a replaceable insert body, which is possible to fix in an insert seat of the basic body by way of a tightening screw. The insert body is ring-shaped and has at least one radially projecting nose on which an operative cutting edge is formed. This means that the most straining force from the workpiece, viz. the tangential force, is applied to the nose, the same aiming at rotating the ring body around the center axis of the tool. The turning insert and the insert seat include co-operating coupling elements in the form of three equidistantly spaced-apart ridges on the insert body and the same number of recesses or grooves in the insert seat, which is located in an end surface of the elongate basic body. Each ridge as well as each recess includes a pair of oblique flanks. However, in this case, the angle between the flanks of each pair of flanks is acute (according to the example, approx. 45°) in order to, in the best way, counteract rotation of the insert body in relation to the insert seat. This means that the coupling elements are absolutely unsuitable to carry axial forces of any importance. In other words, it would not be possible to successfully use the described coupling elements for the transfer of considerable, axially acting cutting forces from a turning insert to an insert seat. Another aggravating drawback of the known turning tool is that the insert body in question is ring-shaped in order to allow the passage of a front-mounted tightening screw. This means that the insert body is considerably weakened due to the central hole, something that in practice precludes a realization of the insert body in the form of a turning insert having a small diameter. In this context, it should also be pointed out that the insert body according to DE 3448086 C2 does not have any endless cutting edge at all.
Coupling elements that include three equiangularly arranged ridges and grooves, respectively, are further known by WO 03/097281 and WO 03/097282. However, also in these cases, the coupling elements have the purpose of providing a rotary locking between an insert body and a basic body, and therefore the angle between the flanks of each associated pair of flanks is acute.
What is more, because the tool is a milling cutter, i.e., a tool the basic body of which rotates during the chip removing machining, the flanks are asymmetrically arranged with the purpose of carrying unidirected rotary motions.